Tight Condition when Burning Off Fuel

[JOJOMO]

When watching NASCAR races on TV, they say that as the car burns off fuel it tightens up (understeer). This seems counterintuitive to me. I would think that you would see more oversteer as your rear end lightens up due to emptying the gas tank.

Could anyone explain this to me?

 

[GregLocock]

It can go either way. As the fuel load drops the load on the rear axle drops. Due to the tire load sensitivity this increases the ratio of the grip to the vertical load, so the rear grips harder, so the car pushes more.

That's fine as theories go, but we've all driven cars (or more likely trucks) that do the opposite as well!

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[edstein]

I think a great deal of the understeer experienced with fuel burn off is from decreacing inertia in the rear of the car. The car is balanced with a full load of fuel, so when the fuel is gone the moment of inertia is decreased and the car "tightens up".

 

[Yawing]

I guess when we are talking nascar its high speed, more closer to steady state cornering. So if thats the case its effectively moving the CG forward when the fuel load burns off, meaning the front tyres need to laterally accelerate a larger proportion of the cars mass, meaning more slip angle required so more U/S.

another factor could be the rear raising with less load should give more rear DF so again more U/S.

low speed traction and quick change of direction is a different story as pointed out by fellow posters.

 

[cibachrome]

Its just simply because the LHS tire cornering stiffness climbs with the reduced wheel load. LHS has considerably higher mu than RHS for D-coded tires used in all oval track series. No different than tires on a standard sedan, but different that tires on perf. sport cars (like Corvette, in which case stiffness climbs with added load. Its all about load rating, rim and pressure for the series the tire is developed for. Pressure also plays a part here,too, During the fuel run pressure increases and drops cornering stiffness and aligning moment stiffness. The reduced aligning moment stiffness loosens the car. So, as you can see, its a rather complex interaction between front and rear axle sideslip stiffnesses, but in the end the net understeer increases. Because the axle tire stiffnesses change, the systemn damping drops, hence the loose in, tightoff problem. Effects of tractive force on tire stiffness are a player, too. Keep in mind where the power is on and where it is off and it should be apparent to you what's happening. On speedway and superspeedway runs, the aero loading is a prominent factor,too, hence encouraging drivers to keep their speed up in spite of shortest distance line theory. In other words, keep the front grip up by keeping your Left front tire (highest mu) loads up using a downforce prominent setup.